1. Field of the Invention
This invention relates to a process for making a flexographic printing plate from a photosensitive printing element, particularly a flexographic element having an infrared radiation ablatable layer capable of being selectively removed by a laser beam.
2. Description of the Prior Art
Flexographic printing plates are well known for use in printing, particularly on surfaces which are soft and easily deformable, such as packaging materials, e.g., cardboard, plastic films, etc. Flexographic printing plates can be prepared from photopolymerizable compositions, such as those described in U.S. Pat. Nos. 4,323,637 and 4,427,749. The photopolymerizable compositions generally comprise an elastomeric binder, at least one monomer and a photoinitiator. Photosensitive elements generally have a photopolymerizable layer interposed between a support and a coversheet or multilayer cover element. Upon imagewise exposure to actinic radiation, polymerization, and hence, insolubilization of the photopolymerizable layer occurs in the exposed areas. Treatment with a suitable solvent removes the unexposed areas of the photopolymerizable layer leaving a printing relief which can be used for flexographic printing.
Imagewise exposure of a photosensitive element requires the use of a phototool which is a mask having clear and opaque areas covering the photopolymerizable layer. The phototool prevents exposure and polymerization in the opaque areas. The phototool allows exposure to radiation in the clear areas so that these areas polymerize and remain on the support after the development step. The phototool is usually a photographic negative of the desired printing image. If corrections are needed in the final image a new negative must be made. This is a time-consuming process. In addition, the phototool may change slightly in dimension due to changes in temperature and humidity. Thus, the same phototool, when used at different times or in different environments, may give different results and could cause registration problems.
Thus, it would be desirable to eliminate the phototool by directly recording information on a photosensitive element, e.g., by means of a laser beam. The image to be developed could be translated into digital information and the digital information used to place the laser for imaging. The digital information could even be transmitted from a distant location. Corrections could be made easily and quickly by adjusting the digitized image. In addition, the digitized image could be either positive or negative, eliminating the need to have both positive-working and negative-working photosensitive materials, or positive and negative phototools. This saves storage space and, thus, reduces cost. Another advantage is that registration can be precisely controlled by machine during the imaging step. Digitized imaging without a phototool is particularly well-suited for making seamless, continuous printing forms.
In general, it has not been very practical to use lasers to image the photopolymerizable layer of the elements which are used to prepare flexographic printing plates. The elements have low photosensitivity and require long exposure times even with high powered lasers. In addition, most of the photopolymerizable materials used in these elements have their greatest sensitivity in the ultraviolet range. While UV lasers are known, economical and reliable UV lasers with high power, such as the ion laser, are generally not available. In UV lasers, such as the excimer laser, the laser cannot be modulated quickly enough to create a precise image at high write speeds. However, non-UV lasers are available which are relatively inexpensive, and which have a useful power output and which can be utilized to form a mask image on top of flexographic printing elements.
U.S. Pat. No. 5,262,275 and pending U.S. patent application Ser. No. 08/341,731, (IM-0672B) describe a photosensitive element and a process for making flexographic plates, respectively. The element comprises a support, a photopolymerizable layer, at least one barrier layer, and at least one layer of infrared radiation sensitive material. The process includes the step of imagewise ablation of the layer of infrared radiation sensitive material with infrared laser radiation to form a mask. The element is then exposed to actinic radiation through the mask and treated with developer solution. This method requires the photosensitive element to contain a barrier layer between the photopolymerizable layer and the infrared sensitive layer. This barrier layer complicates the manufacturing process and increases costs for producing laser ablatable flexographic plates.
Patent application publication WO 94/03839 discloses a photosensitive element and a process for preparing a relief image. The element includes a support, a photosensitive layer, at least one infrared radiation sensitive layer which is substantially opaque to actinic radiation; and a coversheet. Optionally, a barrier layer is interposed between the photosensitive layer and the infrared-sensitive layer. When the element is not imaged and developed immediately after it is put together, it is preferred that a barrier layer be present. The infrared-sensitive layer has an adhesion balance between the coversheet layer and the photosensitive layer or barrier layer such that it adheres more strongly to one of the layers. The process includes imagewise exposing the element to infrared laser radiation through the coversheet. After exposure to infrared laser radiation the infrared-sensitive layer adheres more strongly to the layer to which it had lower adhesion prior to the exposure. The coversheet is then removed having adhered thereto part of the infrared radiation sensitive layer, creating an actinic radiation opaque mask on the photosensitive layer (or barrier layer). The element is then exposed overall to actinic radiation through the mask and developed to form a relief. In this application imagewise exposure to laser radiation changes the adhesion of the infrared sensitive layer.
EP 0 634 695 A1 discloses a laser-imageable flexographic printing plate in which a slip layer doped with a UV absorber is laminated to a photopolymer layer. The slip layer is ablated from the photopolymer layer using a laser operating at a wavelength between 300 to 400 nm to create an in situ negative. The uncured plate is then flood-exposed to UV light in the usual fashion and developed. One disadvantage is that since both the slip layer and the photopolymer layer are sensitive to UV radiation, the laser must be carefully controlled to only selectively ablate the slip layer containing the UV absorber from the UV sensitive photopolymer layer without detrimentally affecting the photopolymer layer itself.